Father. Dr. Derham thought the greatest elocity to be about 60 miles per hour. But we have tables calculated to show the orce of the wind at all velocities from 1 to 00 miles per hour.

Charle.. Does the force bear any geneal proportion to the velocity?

Father. Yes it does: the force increases as the square of the velocity.

Emma. Do you mean, that if on a piece of board, exposed to a given wind, there is pressure equal to 1 pound, and the same board be exposed to another wind of double velocity, the pressure will be in this case 4 times greater than it was before?

Father. That is the rule. The following short table, selected from a larger one out of Dr. Hutton's Dictionary, will fix the rule and facts in your memory.

NOTE.-Mr. Brice discovered, from observations on the clouds, or their shadows moving on the surface of the earth, that the velocity of wind in a storm was nearly 63 miles in an hour, 21 miles in a fresh gale, and nearly 10 miles in a breeze.

CONVERSATION XXXVIII.

Of the Steam-Engine.

FATHER. If you understand the principle of the forcing-pump, you will easily comprehend in what manner the steamengine, the most important of all hydrostatical machines, acts.

Charles. Why do you call it the most important of all machines? it is not a com

mon one.

Father. Steam-engines can be used with advantage only in those cases where great power is required. They are adapted to the raising of water from ponds and E e

VOL. II.

wells; to the draining of mines; and perhaps without their assistance we should not at this moment have the benefit of coal. fires.

Emma. Then there cannot be two opinions entertained respecting their utility. I do not know what we should do with out them in winter, or even in summer, since coal is the fuel chiefly used in dressing our food.

Father.

Our ancestors had, a century ago, éxcavated all the mines of coal as deep as they could be worked without the assistance of these sort of engines. For when the miners have dug a certain depth below the surface of the earth, the water pours in upon them from all sides; consequently they have no means, of going on with their work without the assistance of a steam-engine, which is erected by the side of the pit, and being kept constantly at work, will keep it dry enough for al practical purposes.

The steam-engine was invented during

the reign of Charles II., though it was not brought to a degree of perfection sufficient for the draining of mines till nearly half a century after that period.

Charles. To whom is the world indebted for the discovery?

Father. It is difficult, if not impossible, to ascertain who was the inventor. The marquis of Worcester described the principle in a small work entitled "A century of inventions," which was published in the year 1663, and was reprinted a few years since in London.

Emma. Did the marquis construct one of these engines?

Father. No; the invention seems to have been neglected for several years, when captain Thomas Savery, after a variety of experiments, brought it to some degree of perfection, by which he was able to raise water, in small quantities, to a moderate height.

Charles. Did he take the invention from the marquis of Worcester's book?